re.replace("foo", re"", "-") returns -foo, inconsistent with nre.replace

[kaushalmodi]

Detailed comment: #9315 (comment)

/cc @skilchen

EDIT by @timotheecour

import re
echo replace("foo", re"", "-")
# -foo

• nre returns "-f-o-o-"
• strutils.replace returns "-f-o-o-"
• Python/JS/whatever also does it this way (see Proposed solution for issue #8919 #9280 (comment))
• nim-regex "-f-o-o" but pending Unexpected replacements for "^", "$", "" nitely/nim-regex#29 (which seems accepted), shall return "-f-o-o-"
• strutils.multiReplace currently crashes (strutils.multiReplace() crashes if search string is "" #9557)
• count throws on empty string (since Proposed solution for issue #8919 #9280)

copying over the comment from @skilchen :

import re
echo replace("foo", re"", "-")

produces:
-foo
While in the common understanding of regular expressions there is an empty string in front of any string, one between every substring of length 1 and one at the end of the string.
nre.nim produces the usually expected -f-o-o-
With nre as well as in the other languages I know the following is also possible:

import nre
echo replace("foo", re"^|$", "-")

which produces
-foo-
as expected, but using re you get: -foo which doesn't seem correct.

import re
echo "".replacef(re"([^\d]*)(\d*)([^\w]*)", "$1 - $2 - $3")

still produces an empty line, which doesn't seem correct, I would expect - -. However on a string that usually should be split using this pattern, it works:

import re
echo "abc123###".replacef(re"([^\d]*)(\d*)([^\w]*)", "$1 - $2 - $3")

correctly produces:
abc - 123 - ###

---

further discussion

• 8955470#r30894413
  @Araq said:

There is no wrong logic here, we don't claim to be bug-compatible with other regex engines.

if we're departing from other regex engines, an option could be to throw ; but at least it'd be nice to have consistent behavior across re,nre,nim-regex

[krux02]

Can you please copy the detailed comment here into the issue where it is visible.

And please don't use imperative in the issue title, call the problem it present tense. This also applies to your other issues.

[timotheecour]

@krux02 done (@kaushalmodi I took the liberty to edit your post)

[timotheecour]

@xflywind thanks for self-assigning this; maybe also define -d:nimLegacyReReplace for backward compatibility in your PR?